Tape drives typically utilize an actuator mechanism to position the read/write head over the appropriate tracks while the tape is moving. Current read/write head positioning devices used in magnetic tape drives often incorporate a dual stage actuator design. One actuator provides coarse positioning to move the read/write head between data bands. The other actuator provides fine positioning to maintain alignment between the read/write head and the data tracks. In use, the coarse positioning actuator first moves the read/write head to the general vicinity of the data track on the tape and then the fine positioning actuator is used for track following while the tape is in motion. The two actuators may be mounted in a “piggyback” arrangement with the fine position actuator riding on the coarse position actuator.
The fine positioning actuator is typically a voice coil motor (VCM) mounted on the linear stage and held at a rest position by some type of spring. A VCM actuator provides micron to submicron precision positioning at a bandwidth of hundreds to thousands of hertz. However, a single voice coil and spring combination that can meet the fine positioning requirements across the full width of the tape is expensive and unnecessary. Accordingly, virtually all current tape drives use some combination of a coarse positioning actuator and a fine positioning actuator.
The coarse positioning actuator is typically a linear actuator driven by a stepper motor. Stepper motors have the ability to move the linear stage anywhere across the width of the magnetic tape at modest speeds.
Linear actuators have been designed where a motor drives a threadedly coupled nut (helical gear) which surrounds a corresponding threaded shaft (leadscrew). FIG. 1 depicts a typical helical gear actuator 100. As shown, a helical gear 102 is driven by a threaded shaft 104, which would be driven by a motor (now shown), such as a stepper motor. The helical gear 102 translates laterally as the threaded shaft 104 rotates, thereby carrying the housing body 110 along with it. Through the use of a bias spring (not shown), one end of the helical gear 102 is usually kept in contact with one end of one of the bushings 106, 108 to ensure positive axial retention while the actuator body 110 is being moved along the threaded shaft 104.
FIG. 2 illustrates a typical helical gear 102 as used in current tape drive systems. As shown, the helical gear 102 has a cylindrical shape and flat contact surfaces 202 that typically engage the bushings 106, 108 (FIG. 1) and/or bias spring. The contact surface 202 provides axial control of the actuator body 110, but very little radial control. Thus, some mechanism must be present to avoid wobble of the actuator body 110 relative to the threaded shaft 104.
With continued reference to FIG. 1, the bushings 106, 108 are designed to engage the threaded shaft 104 in order to reduce wobbling. More particularly, each bushing 106, 108 is coincident with the threaded shaft 104, using the outer diameter of the threaded shaft 104 as the datum for proper alignment of the threaded shaft 104 relative to the helical gear 102.
One problem with using the outer diameter of the threaded shaft as the alignment datum is that the threaded outer surface of the threaded shaft is not uniform. Rather, varying thread height results in a varying axial sliding interface as the actuator bushings slide along the outer diameter of the threaded shaft. This can translate into inconsistent coarse motion of the actuator assembly.
In addition, a recurring problem with using bushings is that the bushings, which contact the rotating threaded shaft, tend to wear and create particles. If the threads have minor defects or are non uniform, the threads of the screw can act as a saw on the bushings. The resultant particles have been found to interfere with operation of the actuator. Particularly, the particles tend to accumulate in the actuator, causing it to lose response time and even completely stop functioning in some systems. Further, as the bushings are worn away, wobbling or radial play increases.
Therefore, there is a need to eliminate problems with the guide bushing-to-threaded fastener interface, such as, but not limited to, wear between the bushing inner diameter and threaded shaft outer diameter as a result of axial sliding over an inconsistent surface, and inconsistent coarse motion of the actuator assembly as a result of variations in the thread peaks along the axial length of the threaded shaft.